A digital amplitude modulator is used in a transmitter for, e.g., medium-frequency broadcasting. The digital amplitude modulator includes a plurality of power amplifiers arranged in parallel. The plurality of power amplifiers are turned on or off in accordance with the voltage amplitude level of a signal to be modulated. Consequently, the number of power amplifiers to which amplified carrier signals are output is changed. An AM wave (Amplitude Modulation Wave) is generated by synthesizing the output signals from power amplifiers in an ON state. The generated AM wave is transmitted to a predetermined broadcasting service area.
In the above-mentioned digital amplitude modulator, a transformer using a plurality of toroidal cores is used in a synthesizer for synthesizing the outputs from the plurality of power amplifiers. The synthesizer changes the output power from the power amplifier by changing the turn ratio of the transformer. A serial addition transformer has a structure in which a core is sandwiched. To manufacture this transformer, it is necessary to fix a plurality of cores by positioning them beforehand.